The present invention relates generally to magnetic resonance imaging (MRI) technology, and more particularly, to an apparatus and method to optimize imaging of the peripheral vasculature.
Arteries are the blood vessels emanating from the heart that supply the necessary nutrients to the organs and tissues of the human body. A narrowing or constriction of an artery reduces the delivery of nutrients, such as oxygen to the recipient tissue and has profound effects on tissue function. In general, significant narrowing of an artery leads to compromised function of the organ in question, at best, and organ failure or death at worst. Stenosis or narrowing at any number of locations along the course of the arteries from the abdominal aorta through the calf can result in compromise of arterial blood flow to the distal lower extremities. The evaluation of the peripheral vessels is further complicated by the high incidence of tandem or synchronous lesions, any one of which could be the underlying cause for diminished arterial blood flow. Furthermore, the surgical decisions for potential bypass procedures to improve distal blood flow are greatly affected by the ability to assess the arteries in the foot. As a result, the successful imaging of the lower extremities (i.e. the peripheral run-off study) requires not only the accurate assessment of the presence and functional significance of a narrowing, but also the ability to evaluate the entire arterial course of the peripheral arterial tree from abdominal aorta to the foot.
There are many techniques available for the assessment of the peripheral arteries that include traditional invasive catheter angiography and ultrasound. Because conventional x-ray angiography requires catheterization and the use of nephrotoxic iodinated contrast agents, it is reserved as the final option. Screening for peripheral arterial occlusive disease (PAOD) is typically performed using non-invasive methods such as ultrasound or plethysmography. However, neither of these techniques can provide angiographic illustration of the vessels and merely provides the assessment of individual segments of the intervening arterial anatomy. Both techniques are operator dependent and have confounding technical difficulties which make the imaging often tedious to perform. Moreover, neither technique can provide the comprehensive information required for surgical planning and traditional x-ray angiographic depiction is generally required as an adjunct for pre-operative management.
Magnetic resonance imaging is a method for the non-invasive assessment of arteries. MRI utilizes radio frequency pulses and magnetic field gradients applied to a subject in a strong magnetic field to produce viewable images. When a substance containing nuclei with net nuclear magnetic moment, such as the protons in human tissue, is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field (assumed to be in the z-direction), but precess about the direction of this magnetic field at a characteristic frequency known as the Larmor frequency. If the substance, or tissue, is subjected to a time-varying magnetic field (excitation field B1) applied at a frequency equal to the Larmor frequency, the net aligned moment, or xe2x80x9clongitudinal magnetizationxe2x80x9d, MZ, may be nutated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated (as the excited spins decays to the ground state) and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting MR signals are digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
The imaging of blood vessels using MRI, or magnetic resonance angiography (MRA), is an emerging method rapidly supplanting other non-invasive methods for arterial illustration. Until recently, the application of MRA has been tailored to individual smaller vascular territories (40-50 cm fields of view). With the ability now to translate the table and to image several overlapping fields-of-view or xe2x80x9cstationsxe2x80x9d in rapid succession, MRA can now be used with a bolus chasing technique and result in the imaging of a much larger anatomic length, as necessary for evaluation of PAOD. The rapid performance of multiple MRA acquisitions in sequential and contiguous fashion during the arterial passage of a contrast agent down the lower extremities can yield the depiction of 1-1.2 meters of arterial anatomy. This technique called bolus chase peripheral MRA typically requires 1-2 minutes and utilizes an intravenously administered contrast agent, typically an extra-cellular Gadolinium (Gd)-chelate contrast agent.
This technique relies on the ability to coordinate the acquisition of image data (i.e. the MRA scan) with peak arterial concentration of the contrast bolus that typically occurs during the initial arterial phase of the contrast bolus (i.e., first pass acquisition). Poor coordination of image acquisition (i.e. poor timing) will result in insufficient arterial signal and poor arterial illustration. If imaging is performed late, there can be significant venous and background tissue enhancement that will also diminish the conspicuity of the arterial structures that are already faint secondary to lowered contrast agent concentration.
The typical bolus chase MRA starts in the mid-abdomen and includes the pelvis and extends to the ankle and feet. Timing is typically predicated by the contrast arrival in the initial station (i.e. abdominal aorta) with subsequent imaging performed in automatic sequential xe2x80x9crapid-firexe2x80x9d fashion. Thus, the primary imaging target is the abdominal aorta (i.e. proximal station) and this technique assumes that the speed of imaging alone will allow preferential arterial depiction of all subsequent imaging stations (e.g. thigh, calf and foot). This has been shown to work sufficiently for imaging the peripheral arterial tree above the knee using traditional extra-cellular contrast agents injected at a relatively slow rate, such as in the range of 0.3-1.0 mL/sec. The disadvantage of using a slow infusion rate is that the maximum achievable arterial concentration of contrast agent is markedly diminished and arterial enhancement is often insufficient for reliable depiction of smaller vessels of the infrapopliteal (below the knee) peripheral arterial tree.
Unlike traditional extracellular Gd-chelate contrast agents, intravascular contrast agents persist within the vasculature much longer secondary to their diminished leakage out of the vessels. These contrast agents, therefore, provide an improved opportunity to illustrate vascular structures by maintaining a reliably high concentration of contrast agent within the arteries for an extended period of time, thus providing a prolonged arterial phase or period of arterial enhancement. However, venous enhancement is also prolonged and can significantly diminish the conspicuity of adjacent arterial structures. Moreover, in using intra-vascular contrast agents, venous signal enhancement has an even higher likelihood as there is less dispersion of the contrast bolus into the peripheral tissue extra-cellular space. Therefore, in order to image the entire length of the peripheral vasculature using intravascular contrast agents, venous contamination is a much larger concern than when extra-cellular agents are used since the venous signal can be significant and more persistent. Furthermore, venous contamination is most noticeable in the distal extremities where the arteries are relatively much smaller and fewer in number than their associated venous structures. For example, arterial depiction in the foot and calf can be profoundly complicated by venous overlap when using intravascular contrast agents for peripheral MRA.
It would therefore be advantageous to develop a multi-station data acquisition technique to image the peripheral vasculature in which the distal arteries are well depicted, but at the same time acquiring MR data that is sufficient to reconstruct images to visualize the remaining proximal arterial structures.
The present invention relates to a method and apparatus for optimal imaging of the peripheral vasculature using a contrast agent to emphasize distal arterial visualization during a multi-station examination using MR technology that solves the aforementioned problems.
The invention includes a technique in which data is acquired rapidly in the proximal stations of a multi-station acquisition. The data is acquired with a low resolution acquisition scheme in order to image the most distal station at the optimal arrival time of a contrast bolus. Since the most distal station is the primary imaging target for this particular arterial phase imaging, the data acquired in the distal station is acquired with a high spatial resolution on the first pass of the contrast bolus and at the optimal arrival time of the contrast bolus. The secondary imaging targets of the arterial phase pass of the contrast bolus are the more proximal stations where low resolution images can be acquired with a moving table scheme to chase the bolus to the most distal stations. In this technique the acquisition time for the more proximal stations that include the abdomen, pelvis, and thigh, are chosen to avoid compromising the arterial phase contrast enchancement of the most distal station which can include the lower extremity below the knee or the upper extremities below the elbow. Once the distal station examination is complete, the patient is moved such that higher spatial resolution images at the more proximal stations can be acquired. In this manner, high spatial resolution data from the proximal stations from a subsequent acquisition can either be combined with that from the initial (first pass or arterial phase) acquisitions, or the data from the first pass acquisition can be used to segment out the arterial structures in these proximal stations. This method is well suited, but not limited, to use with intravascular contrast agents that have prolongation of high arterial and venous signal.
The invention includes a method of peripheral vasculature imaging that includes first administering a contrast agent into a blood stream of a patient, then acquiring low spatial resolution MR images of an arterial vasculature of the patient when positioned in a proximal station, and then moving the patient from the proximal station to a distal station. MR data of an extremity of the patient is then acquired at the distal station that is sufficient to reconstruct a high resolution image to visualize arterial structure in the extremity of the patient. The method next includes moving the patient back to the proximal station and acquiring high spatial resolution MR images of the arterial vasculature that may be used alone or can be combined with the low spatial resolution MR images previously acquired in the proximal station.
According to another aspect of the invention, a computer program is disclosed to control a medical imaging scanner. The computer program has instructions to control a computer to move a patient table through a plurality of scan stations that include at least one proximal station and at least one distal station. The patient table is initially positioned in the proximal station and MR data is rapidly acquired after administration of a contrast bolus. The computer program then causes the computer to control patient table movement and move the patient table to the distal station and acquire high resolution MR images therein. Afterward, the patient table is returned to the proximal station to acquire MR image data of a spatial higher resolution than that previously acquired.
The invention also includes an MRI apparatus having a number of gradient coils positioned about a bore of a magnet to impress a polarizing magnetic field, an RF transceiver system, and an RF modulator controlled by a pulse control module to transmit RF signals to an RF coil assembly in order to acquire MR images. The invention also includes a computer programmed to operate the MRI apparatus and after receipt of an indication of contrast bolus passage in a patient, causes the MRI apparatus to acquire low spatial resolution images in a first proximal station, track passage of the contrast bolus, and move a patient table in response thereto after each low resolution image acquisition until MR data is acquired for each proximal station. Then once the patient table is in a distal station, acquire a high resolution image therein. The computer is then programmed to move the patient table back to the first proximal station and acquire high spatial resolution images for each of the proximal stations.
An MR angiography examination is also disclosed that includes administering a contrast agent into a patient, then tracking passage of the contrast agent through the patient, and positioning the patient table such that an arterial structure in a distal portion of an extremity is within a field-of-view (FOV) of the MR scanner. The examination next includes acquiring high resolution images of the arterial structure in the distal portion of the extremity, and then moving the patient such that the remaining arterial structure is within the FOV of the MR scanner. MR images are then acquired of the remaining arterial structure.
The invention is particularly useful with intravascular contrast agents where leakage of the contrast from the vessels is much less than that with extra-cellular contrast agents. By imaging the distal arteries first during a peripheral MRA, using an intravascular contrast agent, sufficient preferential depiction of the distal arteries is achieved. In this manner, arterial visualization is significantly improved for the smaller distal vessels when using a blood pool agent for a multi-station examination.
Various other features, objects and advantages of the present invention will be made apparent from the following detailed description and the drawings.